ROBERT SIEGEL, host:

This is ALL THINGS CONSIDERED from NPR News. I'm Robert Siegel.

MICHELE NORRIS, host:

And I'm Michele Norris.

And here's a question to chew on. Why is it so hard to lose weight? And why is it even harder to keep the pounds off?

Gina Kolata is a science writer for The New York Times and she decided to try to answer that question by examining America's diet industry. The result is her new book called "Rethinking Thin: The New Science of Weight Loss and the Myths and Realities of Dieting." Kolata says scientists have a pretty good understanding of the factors that lead to obesity; less understood is what it takes to permanently remove weight. They do know that people are genetically predisposed to a range of weight.

Ms. GINA KOLATA (Science Writer, The New York Times; Author, "Rethinking Thin: The New Science of Weight Loss and the Myths and Realities of Dieting"): What scientists have discovered over the years in so many studies, it's hard to imagine. They've discovered that everybody has a range of weights, it's biologically, genetically determined. They even know some of the genes now. And it can range between like 20, 30, 40 pounds even.

But if you go to the very bottom of your range, you're trying to get thinner than your body will allow you to be, your body actually fights back. Your metabolism slows down a lot. So the same food that just yesterday was letting you, kind of, lose weight, all of a sudden, the weight's coming back on. And you get voraciously and incredibly hungry. I mean you are obsessed with food.

NORRIS: You call this a primal hunger.

Ms. KOLATA: It's a primal hunger. It's like thirst. It's like holding your breath. Can you hold it forever? No, your body forces you to take a breath of air. In fact, one scientist said to me, what makes you think that hunger is the same for everyone, and especially if the people who've gotten their weights down so low that their body is fighting back. Hunger for them is not what hunger that I feel when it's lunchtime. It's something much deeper.

NORRIS: In the book, you lay out the science and also, sort of, a close examination of the diet industry through the story of a group of adults who were taking part in an experiment that tries to put the Atkins diet and a low-cal diet to the test. And the scientists wanted to know which diet worked best.

You wanted to examine the subjects, their world, their outlook. Why their efforts to lose weight ultimately failed? I know that this is a long journey for these subjects. But what ultimately did you learn in following them through that test?

Ms. KOLATA: It was really interesting because these - I followed in particular four people who allowed me to use their names and tell their stories in great detail. Most of them had been on every diet you can think of. They've been struggling with their weight their entire life. And yet, every time they lost weight to where they were getting close to where they thought they wanted to look, the weight would come back.

But again, I went to the study saying, two years, the federal government is funding this thing. They're going to throw everything at us, that anybody knows how to give us to help us lose weight. So for the first year, the weight was just kind of falling of. One guy, Carmen, who was a major character in my book, had lost about 65 pounds and he said: I feel good. And he wanted to lose weight like many people do because he was worried about health risks. But he also said: I live in America; you have to be beautiful.

Well, after the first year, the weight started coming back, and they started saying things like, I don't know, you know, I'm just failing. I don't understand it. I just can't stand to stick with it. I had a lapse and I had another lapse. And I'm not staying with the program, then they no longer were writing down everything they ate. Well, no surprise, because, you know, they - why do you want to write down everything you ate when all of a sudden it's a binge.

When the study ended after two years, at least the groups I was studying, almost everybody had gained back almost everything they had lost.

NORRIS: At the end of this experiment, you described the last meeting in March of 2006. Most of the participants in the study didn't show up. And as parting gifts, they were each given a University of Pennsylvania mug and a T-shirt sized extra-extra large. And I wonder what that meant, if there was an expectation in that of the people who were conducting the study knew from the outset that most of them would fail?

Ms. KOLATA: I think that they knew what was happening. That's what's so disturbing about this. When you see the weight loss industry, they're selling a dream just like the cosmetics industry does. You know, they said put this cream on, you're going to look like a 20-year-old, you know you won't, but you buy it anyway.

And the diet industry - there's all these books out there, these anecdotes of people telling about how they lost all this weight and their life was transformed. But every time that there's a study that's actually done in a rigorous way, almost everybody gains back almost everything that they have lost.

NORRIS: So with all of this research, with that biological drive, the power of genetics, why do we see so many diets, diet after diet - the South Beach diet, the Atkins diet, the grapefruit diet. They advise to take all the whites from your diet - the carbs, the sugar, any kind of white flour. How much of the American diet industry is boosted by actual science and how much of it by marketing forces?

Ms. KOLATA: I think it's all marketing forces. But I think there's a reason why it persists. There's a psychologist I spoke to when I was writing my book and he answered a really good question. He said in psychology if you keep on punishing a behavior, an animal or a person stops doing it. If you give a rat an electric shock every time it tries to eat some rat chow, it's going to stay away from that bowl of rat chow.

So why is it that when people go on the grapefruit diet and then that fails, and then they go on to the Atkins diet, then they go on a low-calorie diet, whatever, at the end, they come back to their old weights.

And he said in studying it, he discovered that there's a real reward in dieting that you feel great when you start. You know, the weight is falling off. You look good. Everybody's complimenting you. And then, of course, they plateau and the weight starts coming back and they start having these uncontrollable urges to eat. And instead of saying, boy, you know, I'm never going on a diet again, was that ever a punishment. They say: I fail, I have stress at work, or whatever, something made me go off that diet. But then they see another diet and the magic is there again, they want to start.

NORRIS: So I just want to make sure I understand the overall, you know, message that someone might take from the book. For someone who, say, 75 or 100 pounds overweight, you're not saying that they shouldn't try to diet, but they should understand what they face when they do embark on that journey.

Ms. KOLATA: If somebody is 75 or 100 pounds overweight, yeah, they should understand that the odds are against them ever becoming thin, thin. They can lose some weight and they may be a lot healthier but it's extremely unlikely that they're going to be as thin as fashion dictates.

But you have to be realistic and you know when you've gotten too thin for your body because you will start finding it impossible to control your eating and it's not your fault.

NORRIS: The book is called "Rethinking Thin." I'm wondering if we should be rethinking that just being thin but the use of the word, thin. Maybe perhaps the book should be thinking about getting healthier instead of getting thin.

Ms. KOLATA: I would love it if that would happened and I wish that people who are struggling with their weight weren't disparaged so much and told it's all their own fault.

NORRIS: Gina Kolata, good to talk to you.

Ms. KOLATA: Thank you, Michele. Good to talk to you.

NORRIS: Gina Kolata is the author of "Rethinking Thin: The New Science of Weight Loss and the Myths and Realities of Dieting." And at npr.org, you can read an excerpt from her book. It includes more about Carmen, the man on the study, who just couldn't stick to his diet.

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